This invention relates to electronic means for controlling mechanical resonances in closed loop servo systems. In particular, the invention is described with reference to a servo system used to drive magnetic read-write heads radially with respect to rotatable magnetic disk storage media, and comprises a method whereby mechanical resonances occurring in the servo drive system can be prevented from causing instabilities in the servo loop leading to inaccuracies and/or loss of speed of response of the servo system.
It is presently common practice in the data processing industry to store large quantities of magnetic data in digital form upon rotating disk storage media, i.e. magnetic disks. Such disks are of numerous types; the present invention pertains to rigid (i.e. non-floppy) magnetic disks, a plurality of which are stacked atop one another on a common shaft and rotated at a steady speed. Magnetic read/write heads, one for each side of each disk, are moved in and out radially with respect to the disks in response to machine commands. All the magnetic heads are mounted on a single head drive means and moved together, until the head is positioned at the distance from the center of the disk where the particular record sought to be read or written is located. Clearly it is desirable to provide a magnetic storage media with as much data storage capacity as possible. To this end, naturally it is desirable to locate the tracks as close to one another as possible. The drive means used to move the magnetic heads radially with respect to the disks must be correspondingly accurate, regardless of the type of drive chosen. The usual drive choice is a servo system in which a motor is used to drive the magnetic head mounting structure back and forth with respect to the disk and in which position signals relative to the actual position of the heads are generated by a moving position sensing head interacting with permanently coded position information on the disk itself, so as to provide an accurate position sensing means. The information gained from this position sensing head is then used to generate a desired position command, used to control the future action of the motor moving the magnetic heads radially with respect to the disk. Such a "feedback" servo system is well known in the art. However, the high performance requirements of modern data storage media pose considerable difficulty. It is desirable to treat magnetic disk storage media as "random access memory"--that is, memory which may be non-sequentially accessed--which requires that the access time of the magnetic disk storage media be as short as possible. If this facility is to be economic with today's high speed computers, the servo system must be increasingly accurate and fast. This in turn leads to difficulties with mechanical resonances in the servo system, including the heads themselves. In order that the data may be packed densely on the magnetic disk, it is necessary that the heads "fly" on the magnetic surface on an air bearing on the order of several millionths of an inch thick. Clearly, only a minor resonance is required to destroy such a delicate air bearing and it is therefore essential that mechanical resonances be avoided completely. A further criterion which must be considered in detail by the designer of the servo system is accurate position sensing of the heads relative to the disks so that the data can be reproducibly read or written onto specific areas of each individual disk. Numerous systems have been proposed in the prior art for generating accurate position sensing information, see, e.g., U.S. Pat. No. 3,820,712 to Oswald for "Electronic Tachometer" which is in some ways similar to the system employed by the present invention and upon which the present invention may be to some extent construed as an improvement. See also U.S. Pat. Nos. 3,568,059 to Sordello for "Electronic Tachometer" and 3,833,894 to Johnson for "Disk Drive Servo System" which are somewhat less relevant approaches to similar problems.
All three of the systems described in the patents mentioned above operate using the same source of position signals. One of the plurality of magnetic disks mounted on the central spindle for rotation is provided with regularly spaced magnetic codes which can be detected by a magnetic head and counted in order to provide accurate position sensing information. The signal generated by this head is essentially a sine wave as the head is moved towards a new position during a "seek" operation (hereinafter sometimes a "seek"). Such a sine wave signal is useful for counting the intervals between such permanently coded magnetic bands but in itself is not suitable for controlling a servo motor, as the sine wave is non-linear at positions not near its nulls (i.e., near the 90.degree. and 270.degree. peaks of the signals) which tends to cause difficulty of control. To avoid this problem, the prior art as exemplified by the Oswald patent referred to above indicates that a control system may operate part of the time on a signal produced by differention of the sine wave produced by the magnetic position sensing head and the remainder of the time on a signal derived from the current drawn by the servo motor as it moves during a seek. This motor current signal may be integrated to give a signal proportional to the velocity of the servo system and thus an indication, over time, of the position of the servo-controlled apparatus. However it will be understood that a velocity signal derived from motor current will ignore certain variables which tend to influence velocity, such as windage, bearing wear and the like and that therefore an actual position sensing signal is needed to accurately control the motion of the servo driven apparatus. In the Oswald scheme, as mentioned above, the integrated velocity signal is used as the primary control but is updated periodically by replacing the initial conditions used in the integrating process by position signals derived from the magnetic position sensing means. In a preferred embodiment, according to Oswald, the position information (i.e. the initial conditions of the integration) is updated every 180.degree. of the sine wave signal, at 30.degree. before and after the nulls of the sine wave, i.e. when the amplitude of the wave is at half its maximum value. This approach is used in the present application under certain circumstances to be detailed more fully below.
Thus, while the combination of the differentiated position signal and the integrated current or velocity signal according to Oswald yields accurate velocity sensing, Oswald does not solve the problem of mechanical resonance. In a typical disk drive system, the servo is first directed to accelerate the apparatus being driven to a given velocity, then to hold that velocity until the desired point of reading or writing is approached, and finally to decelerate the apparatus as quickly as possible until the desired point is finally reached, when the servo is directed to halt the apparatus. It has been found that mechanical resonances in the system during the steady velocity phase of the seek are aggravated by any instabilities in the servo control loop and, as mentioned above, these mechanical resonances are most damaging to the system performance. In the prior art, therefore, the mechanical resonance problem has been overcome by limiting the bandwidth of the servo loop, essentially lowering the magnitude of the resonant peak in the mechanical resonance system below unity gain so that the servo loop would not resonate in "tune" with the mechanical system. The difficulty with this approach is that the loss of bandwidth in the servo loop causes a larger error in the output--that is, erratic arrivals of the magnetic read/write head and therefore a loss of reliability. Alternatively, reliability can be increased by slowing down the overall motion of the servo system, but of course that is not desirable either.